This invention relates to a control system for an electric power supply apparatus, and more particularly to a circuit for controlling an electric power supply apparatus, a method therefor and an uninterruptible power supply apparatus suitable for use in promptly and stably controlling a controlled variable of an output voltage, an output current, an output power and the like of an electric power supply apparatus.
First, an uninterruptive power supply, which is utilized to prevent troubles from occurring in computers, medical instruments and the like in the event of a power failure, will be explained as an example of a prior art electric power supply apparatus. The uninterruptible power supply is adapted to supply a load such as a computer, a medical instrument and the like with a stable alternating current (AC) electric power, generally from a commercial electric power network, and, in the case of a power failure, from a direct current (DC) electric power generated by a storage battery, and comprises an inverter for converting a DC electric power to an AC electric power and an output filter for removing higher harmonic voltages generated by the inverter from an output voltage waveform.
However, the above-mentioned computer, medical instrument and the like are generally loads having nonlinear impedance characteristics, which results in generating distortion in an AC output voltage waveform delivered from the uninterruptible power supply. Also, the AC output voltage may be decreased due to a sudden change of the load. Since such distorted waveform and voltage decrease may cause a malfunction and a trouble of a computer, a high speed control for effectively suppressing a distorted waveform and a voltage decrease in an AC output voltage waveform is now under investigation.
As such high speed control system for an AC output voltage waveform, there are known techniques described in the documents "Materials for 68th Meeting of Power Electronics Society" and "Materials SPC-87-9 for Power Electronics Society (1987)".
These prior art control systems employ a dead beat control and therefore are effective in a high speed control of an AC output voltage waveform. However, as described in the above-mentioned documents, it is necessary for achieving a dead beat control characteristic to compensate for a time delay caused by a processing time required by a processing means employed in a control circuit, a delay caused by a delayed response from a detecting circuit which takes in a controlled variable and the like, thereby making it difficult to build a stable control system.
Further, when a pulse width modulation type inverter is employed in an electric power converting apparatus, such pulse width modulation type inverter generally controls to turn on and off switching elements comprised in the inverter so as to output a voltage having a pulse width in proportion to an output voltage reference value. Also, the on-off control of the switching elements is performed by providing the switching elements with an on-delay for preventing a power supply short.
Such pulse width modulation type inverter generates an unstable output voltage during an on-delay period, which may result in a problem that an output voltage waveform is distorted.
To solve such problem of the pulse width modulation type inverter, there are known techniques, for example, described in "Anthology of Electric Academy Papers" Vol. 107-D, 2, pages 183-190, published in 1987, and the like.
This system prolongs a pulse width modulated signal by a predetermined value when an output current has the positive polarity and reduces the same by a predetermined value when the output current has the negative polarity, to thereby adjust the voltage pulse width to be an essentially necessary width.
When an AC output voltage waveform of an uninterruptible power supply is controlled by the above-mentioned prior art system, it is necessary to control the AC output voltage waveform at a high speed in addition to compensating for influences caused by a delay due to a processing time of a processing means employed in a control circuit, a delay due to a response delay of a detecting circuit which takes in a controlled variable, and the like, thereby making it difficult to fullfil a high speed response control system implementing the above-mentioned dead beat control (finite stabilizing control) and the like susceptible to influence of delay.
Further, a prior art on-delay compensating method for pulse width modulation type inverter is problematic in that when a current flowing into a load of the inverter include a large ripple, a region in which the polarity of the current is reversed exists within a half period of a carrier, so that there exist, in a single carrier period, an on-delay period in which an output current has the positive polarity and another on-delay period in which the output current has the negative polarity. It is therefore impossible to perform a precise on-delay compensation and accordingly improve an output voltage waveform.